In computer tomography, tomographic recordings of a subject, in particular a patient, are generally made with the aid of absorption measurements of X-rays which pass through the subject, a radiation source generally being moved circularly or spirally around the subject and a detector on the opposite side from the radiation source, usually a multiline detector with a multiplicity of detector elements, measuring the absorption of the radiation when it passes through the subject. For tomographic image compilation, tomographic section images or volume data are reconstructed from the measured absorption data of all measured geometrical rays. Absorption differences in objects can be represented very well by these computer tomographic recordings, but regions with similar chemical composition, which naturally also have a similar absorptivity, can be represented only with insufficient detail.
It is furthermore known that the effect of the phase shift when a ray passes through a subject is substantially stronger than the absorption effect of the matter through which the radiation has passed. Such phase shifts are measured in a known way by using two interferometric gratings. With respect to these interferometric measurement methods, reference is made for example to “X-ray phase imaging with a grating interferometer, T. Weitkamp et al., 8th Aug. 2005/Vol. 12, No. 16/OPTICS EXPRESS”.
In this method, coherent X-radiation passes through a subject, the X-radiation having passed through is guided through a grating pair and the radiation intensity is measured immediately after the second grating. The first grating generates an interference pattern, which forms an image of a moire pattern with the aid of the second grating on the detector lying behind. If the second grating is displaced slightly, then this likewise causes a displacement of the moire pattern i.e. a change of the local intensity in the detector lying behind, which can be determined relative to the displacement of the second grating.
If the intensity change is plotted for each detector element of this grating, i.e. for each ray, as a function of the displacement distance of the second grating, then the phase shift of the respective ray can be determined. A problem, making it unsuitable for carrying out computer tomography of sizeable objects, is that this method requires a very small radiation source since coherent radiation is needed for forming the interference pattern.
The method presented in the document cited above requires either a radiation source with an extremely small focus, so that there is a sufficient degree of spatial coherence in the radiation used. When using such a small focus, however, then a sufficient dose power for examining a sizeable object is in turn not available. It is nevertheless also possible to use monochromatically coherent radiation, for example synchrotron radiation as the radiation source, but this makes the CT system very expensive to construct so that widespread application is not possible.
This problem can be circumvented by arranging a first absorption grating inside the focus/detector combination in the beam path, immediately after the focus. The alignment of the grating lines is in this case parallel to the grating lines of the interference grating which follows after the subject.
The slits of the first grating generate a field of individually coherent rays with a particular energy, which is sufficient for generating the interference pattern known per se with the aid of the phase grating arranged behind the object in the beam direction.
In this way, it is possible to use radiation sources which have extents that correspond to normal X-ray tubes in CT systems or transmitted-light X-ray systems so that, for example, even well-differentiated soft tissue tomographs can now be made in the field of general medical diagnosis.
A problem with this type of focus/detector combination is that a plurality of measurements need to be carried out respectively with a slightly displaced analysis grating for each ray, so that it is possible to determine the phase shift of the X-radiation on the respective ray path through the subject.